Marking or dulling of tin plate



United States Patent 2,876,176 MARKING on DULLING F TIN PLATE Application January 25, 1956 Serial No. 561,353

13 Claims. (Cl. 204-37) No Drawing.

This invention relates to the marking of tin plate and more particulary to a process and composition for providing an identification marking on one side of differentially coated tin plate.

The production of tin plate by the electrolytic tinning method involves the steps of passing a continuous ribbon of strip steel through a tank containing a solution of a tin salt and therein electrodepositing tin on the steel strip. After leaving the tinning tank, the freshly plated strip passes through a furnace or oven at a temperature sufiiciently high to fuse the tin in order to form a bright even coating on the steel strip. A relatively recent development in electrolytic tinning is the production of differentially coated tin plate wherein the electrolytic tinning step is so regulated that one side of the steel strip receives a relatively heavy coating of tin and the other side rece ives a relatively lower weight coating of tin. For purposes of tin can manufacturing and the like, arelatively heavy, corrosion resistant coating of tin is requiredonly on the inside of the can. Thus, the use of differentially. coated tin plate avoids an unnecessarily heavy external coating of tin thereby resulting in important economies in the use of tin.

However, with the advent of differentially coated .tin plate in the industry, it Was necessary that the tin plate manufacturers have some means for markingthedilfer: entially coated tin plate which would permitthe can manufacturer to identify the light coated side or otherwise distinguish between the different sides. As the difierena tially coated tin plate is discharged from the fusion step, ofthe electrolytic tinning line, bothsides of the strip normally possess the customary bright highly reflective finish.

Various marking techniques have been proposed and tried heretofore but no entirely satisfactory scheme has been developed prior to the present invention. For example, certain types of printed markings have been tried but these have not been acceptable because the marking or design tended to fade after a period of storage. Another method of identification which has been utilized to some extent involves the use of a roughened roll at the skin mill so that the steel strip prior to introduction to the electroplating bath has one side roughened. The roughened'surface after electrolytic tinning possesses a dull surface appearance or a decreased reflectance as compared with the tin coating on the smooth side of the strip. Although this purely mechanical pretreatment of the strip to effect dulling of one side gives a permanent marking, it has been found that the dulling effect is far from uniform. Furthermore, the corrosion resistance of the tin plate is adversely affected and this, of course, is a serious objection for most uses of tin plate. Moreover, the tin plate producer also finds the technique inconvenient since it necessitates frequent changes of rolls at the skin mill due to the tendency of the rough roll to become smooth after a relatively short period of rolling ice 2 some extent involves treating one side of the steel strip anodically in between the electrolytic tinning-and fusion. steps, but for satisfactory dulling by this means it is also necessary that the base metal strip be roughened at least slightly in the rolling operation prior to electrolytic tinning thereby introducing the same objections of non-uniformity of dulling and reduced corrosion resistance of the tin plate. Moreover, the anodic treatment frequently is ineffective along the marginal edges of the strip so that the over-all dull surface appearance desired by the can manufacturers is not obtained.

Accordingly, it is a primary object of the present inven-' tion to provide novel. and improved means for markingor dulling tin plate.

Another object of the invention is to provide a novel and improved means for providing an identification marking on one side of differentially coated tin plate.

A further object of the invention is to provide a novel and highly satisfactory means for marking or dulling tin plate which does not necessitate preliminary rougheningof the base metal thereby avoiding impairment of the corrosion resistance properties of the tin plate.

Still another object of the invention is to provide a novel step in an electrolytic tinning process for imparting a uniform dull surface appearanceto one side of the tin plated strip. An additional object of the invention isv to provide a novel solution or composition for use in dulling or mark ing tin plate.

Briefly, the present invention involves the step ofapplying to a tin coated surface of the base metal strip intermediate the electroplating and fusion steps an aqueous solution of a neutrol or slightly alkaline salt of an alkalinous metal. The term alkalinous metal, as used. herein, means a metal selected from the well-defined group consisting of the alkali metals and the alkaline earth metals. For best results, the treating solution should also contain a surface active agent, particularly a non-ionic wetting agent, and in many instances the addition of an alkali metal silicate is also highly desirable.

Although the invention is clearly applicable to a batch or semi-continuous tinning operation, it is particularly adapted for use in' 'a continuous electrolytic tinning line in which case the treating solution is most conveniently. applied by means of strategically located sprays intermediate the electroplating bath and the fusion: furnace. The marking technique herein disclosedis useful in any application where a dull tin plated surface .is desired, but ll. is particularly useful in marking one side only of differentially coated tin plate in order to permit the light coated and heavy coated sides of the strip to be distinguished from each other. As hereinafter mentioned, the well known electrostatic coating techniques are par-v ticularly useful in such instance since with this method the over-spray is readily controlled and the marking or dulling is readily confined to only one side of the strip.

In the usual continuous electrolytic tinning line, the strip as it emerges from the plating, tank is passed through a water rinse following which the tin coated strip is then passed through a fusion furnace for effecting fusion of the tin coating to obtain an even bright finish on the tin plate. According to the presentinvention, the marking or dulling solution is applied to the strip prior to entry of the strip into the fusion furnace, i. e.- interrnediate the electroplating and. fusion steps. For marking differentially coated tin plate, it is generally preferred to apply the marking solution to the light weight side of the tin plate which becomes the exterior side of the final tin can or container. It has been found that the uniform dulled surface at the light weight side of the tin plate provides. an excellent surface for the application of decoration 'or printed 'matter by" lithog raphy or other means and in some instances the dulling or marking means of the present invention permits the omission of the conventional background coating used in lithographic decoration of tin containers.

. 'Although any suitable method of applying the markmg solution may be used, spray application is generally the most satisfactory since the relatively large quantities of solution required on the sheet can readily be obtained bythe use of sprays. Ordinary mechanical sprays or atomizers can be utilized, but the preferred method of appllcatlon is by electrostatic coating because of the ease with which overaspray is controlled thereby avoiding contamination of the working area. Moreover, it is extremely important in the case of differentially coated tin plate that the application of the solution be confined to only one side of the strip. In the electrostatic appli cation of the marking solution it is a relatively simple matter to avoid wrap around, i. e. contact of the mark mg reagent with the reverse side of the strip relative to the spray nozzles. Electrostatic coating is well known in the art and need not be described in any detail except to say that the treating solution is converted into a spray by air pressure, or by mechanical or electrostatic atomization, and under the influence of an electrostatic field so. that the spray particles are electrically charged and are attracted to-the strip and deposited thereon. Reference is made to such prior art patents as Ransburg et al. 2,334,648,.Ransburg 2,658,009, and Ransburg 2,- 698,814 for various details of the electrostatic coating methods and devices.

The compounds which may be used for marking in accordance with the invention comprise aqueous solu-' tions of the non-acidic salts of the alkali or alkaline earth metals, particularly, the neutral or slightly alkaline salts of these metals. The strongly alkaline salts are generally undesirable because of their tendency to react with or attack tin. More specifically, the chlorides, bromides, iodides, sulfates, carbonates, nitrates and chromates of sodium, potassium, lithium, rubidium, cesium, calcium, barium, strontium, and magnesium are particularly useful. Because of its cheapness, solubility, and availability, sodium sulfate is the preferred compound. Of course, in addition to the foregoing requirements, it is also necessary that the alkalinous metal salt be stable, i. e. does not decompose, volatilize, or sublime, at the elevated temperature of the strip passing through the fusion furnace so that the salt remains on the surface of the tin plate. during the fusion step. In general, it is necessary thatthe salt be stable slightly above the fusion point of tin (450 F.).

- Although all of the foregoing salts in aqueous solution are effective marking or dulling agents, there is in some cases a tendency for the salt to become displaced or detached from the surface of the tin plate during the extremely rapid heating and evaporation of the solution which takes place prior to and during the fusion step. In commercial practice, the fusion stage of an electrolytic tinning line may comprise either an induction furnace, a resistance heating unit wherein the steel strip is heated internally by electricity, or. a gas fired furnace wherein the fusion of the tin is accomplished by means of radiant heat from gas burners. With certain aqueous salt solutions, and particularly in a high temperature furnace, the rapid rate of evaporation of the solution apparently results in decrepitation or spalling of the dried salt residue so that the particles or crystals of the salt literally "pop off from the strip and the desired uniform marking or dulling effect is not obtained to a satisfactory or reproducible degree. In order to overcome this tendency it has been found that an alkalinous metal silicate may be added to the treating solution with good results. Sodium silicate is the preferred compound because of its cheapness and availability. It is also preferred to use a filtered grade of sodium silicate in order to assure a the exact mechanism of the silicate in controlling decrepitation and spalling of the dried marking salt is not entirely clear, it is believed that the silicate acts as a binder between the dried particles of the marking salt and the tin coated base and also possibly effects a more controlled and less explosive evaporation of water during the heating step.

For effective uniform dulling of the tin plated surface, it is preferred that the alkalinous metal salt be present to the extent of at least about .100 milligrams per square foot of treated surface area. From practical considerations, the preferred range is from about 100 to about 300 milligrams persquare foot of surface area. The concentration of the aqueous treating solution is not particularly important as long as the aforementioned minimum amounts of alkalinous metal salt are deposited on the surface undergoing treatment. However, the alkalinous metal salt is preferably present in a concentration of from about 2 to about 8 grams (dry basis) per 100 cc. of solution, and the alkalinous metal silicate is prefer ably present in an amount of from about .75 to about 2 cc. per 100 cc. of solution. It will be understood that the quantity of aqueous solution applied to a given area of tin plate and the concentration of alkalinous metal salt in the solution are interrelated variables which must be correlated or adjusted in accordance with the requirements of a particular installation in order to obtain the required minimum deposit of 100 milligrams of alkalinous metal salt per square foot of treated surface area. Actually, as far as the concentration of the treating solution is concerned, the maximum upper limit is determined only by the solubilities of the particular salt or salts in the solvent used.

In order to reduce the surface tension of the aqueous marking solution and to insure uniform distribution of the solution as applied to the surface of the strip, it is preferred to include in the treating solution a small amount, on the order of from about 0.1% to about 0.5% by volume of a suitable surface active or wetting agent. It has been found that the non-ionic surface active agents are particularly effective for this purpose, e. g. the polyoxyethylene ethers and the polyethylene glycol alkyl ethers. As the aqueous treating solution on the surface of the tin plate is evaporated during passage of the strip through the fusion furnace, the rapid concentration of the aqueous film results in a tendency for the uniform film to be converted to a dis-' continuous collection of droplets on the surface at an use at the exterior of tin plate containers or the like.

early stage of the evaporation. This wetting or balling up tendency detracts from the desired uniform dulling or marking of the surface, but the presence of the surface active agent minimizes this efiect to a large extent. Although the use of the surface active or wetting agent is desirable in all cases, it is believed that the presence matte appearance which is uniformly dull and has a substantially diminished reflectance as compared with the untreated bright in plate surface. The dulled surface of the tin plate is quite receptive to decorationby lithographic printing or the like and, as previously mentioned, it is frequently possible to omit entirely the usual white or light colored background layer used in lithographic printing. Thus, the treated light coated side of differentially coated tin plate is especially adapted for Moreover, the process of the invention does not necessicontinuously satisfactory spraying operation. Although 15 rate preliminary rough rolling of one side of the base metal strip prior to the tinningoperation. The elimination of the necessity for this preliminary roughening of the base metal strip completely avoids theobjectionable decrease in corrosion resistance of the tin plate which has frequently been encountered with the marking methods heretofore proposed. 7

By way of further illustration of the invention, the following specific'exarnples are presented but it will be understood that the invention not necessarily limited to the compositions and conditions described in these examples.

Example I A number of tests were conducted on a laboratory scale utilizing 3" by 8" samples of rolled strip steel having a matte finish of unfused electrolytically deposited tin in an amount of about Alb. per base box. In each test one of these tin plate samples was sprayed, using either a hand atomizer or a mechanical spray or an electrostatic spray, with an equeous solution at room temperature of a selected alkali or alkaline earth metal salt ashereinafter listed. Solutionsof various concentrations were tried ranging from 0.5 to 10 grams of the salt per 100 cc. of solution, and in each case the spraying operation was controlled to insure coating of the surface of the sample with from 100 to 300 milligrams of the alkali or alkaline earth metal salt per square foot of surface treated. In addition to the alkali or alkaline earth metal salt, each aqueous treating solution also con tained about 0.1% by volume of the non-ionic surface active agent manufactured and sold by Carbide and Carbon Chemicals Co. under the trademark Tergitol TMN which is -a polyethyleneglycol alkyl ether. I Using the foregoing general technique, the test specimens of tin plate were treated with solutions of the following salts:

Potassium bromide Lithium chloride Barium chloride Calcium chloride Strontium chloride Strontium nitrate Rubidium chloride Cesium chloride Magnesium chloride Calcium sulfate Sodium chloride Sodium nitrate Sodium chromate Sodium dichromate Potassium sulfate Potassimum chloride Potassium nitrate Potassium chroma'te' Potassium dichro'rnat'e Potassium iodide In the case ofcalc'ium sulfate, the treating solution was in the form of' an aqueous suspension rather than a solution.

In each case, the test specimen after being sprayed with the treating solution was heated to a temperature somewhat above 450 F. in order to eifect fusion of the tin coating. The fusion was accomplished either by induction heating, gas heating, or resistance heating. Following the fusion step the treated surface of the tin plate was inspected visually and also the reflectance was determined quantitatively by a refiectometer. In each case, a satisfactory uniform dull surface appearance was obtained having a generally whitish quality which was sharply differentiated from the bright highly reflective surface of a standard sample of fused tin plate used for comparison purposes. In addition to the visual and instrumental observation of the dulling obtained, measurements were also made of the surface roughness of the samples using a profilometer. In general, the test samples having a satisfactory dulled surface appearance gave a profilometer reading of from about 10 to about root mean square average. This compares quite favorably with a root mean square average range of from about 35 to about 50 for tin plate specimens which have been subjected to a preliminary surface roughening at the skin mill in accordance with certain dulling or marking methods heretofore proposed. As previously mentioned, the preliminary surface roughening has been cc. per cc. of solution. With gas heating for the fusion step the addition of the sodium silicate was found to be necessary in many instances in order to avoid decrepitation of the dried salt.

Example II The marking method of the present invention was carried out on a full scale commercial electrolytic tinning line producing differentially coated tin plate. The aque-; ous marking solution was applied to the light coated side of the moving strip by means of a Ransburgele'ctro static coating system located in'the line intermediate the electroplating tank and the fusion furnace. tion was applied as a spray at an atomization pressure of 50 lbs. per square inch and under the influence of an electrostatic field obtained by imposing a voltage of 100 kv. on the grid of the system.

The treating material consisted ofan aqueous solution of 5% by weight of sodium sulfate, 1.0 to 1.3% by volume of sodium silicate (having a silicazsoda weight ratio of 3.25:1), and 0.3% by volume of a non-ionic surface active agent comprising a polyoxyethylene ether sold by Monsanto Chemical Co. under the trademark Stero'x AI. At a line speed of 500 ft. per minute using a strip 30 inches Wide, the sprays were adjusted to-deposit approximately 1 gal. per minute of the marking solution on the strip. I i

In order to prevent wrap-around of the treating solution, a backing electrode somewhat wider than the moving strip was located closely adjacent the rear or heavy coated side of the strips This backing electrode was maintained at ground potential so that any over-spray passing beyond the side edges of thestrip was attracted to the backing electrod'e and thus prevented from contacting the rear or heavy coated side of the strip. Following the spraying step, the strip passed continuously through agasfired fusion furnace and the resultant differentially coated tin plate had the. usual bright shiny surface on the heavy coated sideof the strip but was uniformly dull in appearance on the light coated side.

By way of summary, the invention as herein described provides a highly satisfactory method for dulling the sur face 'of tin plate and is especially adapted for use in a continuous electrolytic tinning line producing differentially, coated tin plate. The invention lends itself rea i y. to the use of electrostatic coating techniques and the entire procedure is relatively simple and inexpensive to install and operate. The marking obtained is a uniform dull surface appearance which is highly acceptable to the can manufacturers and is particularly receptive to lithographic printing or the like. The invention results in a permanent marking which does not fade or diminish during storage. Furthermore, the invention completely eliminates any necessity for preliminary surface roughening of the base metal at the skin mill which has been found to result in a detrimental decrease in corrosion resistance of the product as well as irregularity and nonuniformity of dullness. Most of the reagents employed are inexpensive, readily available, and unobjectionable from health and safety considerations to the manufacturers of cans or containers for food products.

We claim:

1. In the production of tin plate by electrolytically depositing tin on a base metal sheet and thereafter heating the sheet to an elevated temperature for fusing the electrodeposited tin, the improvement which comprises applying to a tin coated surface of the sheet intermediate the electrodeposition and fusion steps an aqueous solution containing both an alkali metal silicate and a salt selected from the group consisting of the alkalinous The solu-' metal chlorides, bromides, iodides, sulfates, carbonates, nitrates, and'chromates, said salt being stable at the elevatedtemperature of the fusion step, and the amount and concentration of the solution applied being suificient to provide at least 100 milligrams of said salt per square foot of treated surface whereby the tin plate following the fusion step is provided with a dull surface marking,

and said silicate preventing loss of the salt from the sheet by decrepitation and spalling during the fusion step.

2. In the production of differentially coated tin plate by the controlled electrolytic deposition of tin on a base metal sheet so that opposite sides of the sheet have relatively light and relatively heavy tin coatings, respectively, and thereafter heating the sheet to an elevated temperature for fusing the electrodeposited tin, the method of providing an identifying marking on one side of the sheet which comprises applying'to only one of the tin coated sides of the sheet intermediate the electrodeposition and fusion steps an aqueous solution containing both an alkali metal silicate and a salt selected from the group consisting of the alkalinous metal chlorides, bromides, iodides, sulfates, carbonates, nitrates,

, and chromates, said salt being stable at the elevated temperature of the fusion step, and the amount and concentration of the solution applied being sufficient to provide at least 100 milligrams of said salt per square foot of treated surface whereby to provide on said one side of the sheet following the fusion step a dull surface marking of materially reduced reflectance thereby distinguishing the opposite sides of the sheet from each other, and said silicate preventing loss of the salt-fromthe sheet by decrepitation and spalling during the fusion step.

3. The method of claim 2 further characterized in that said alkalinous metal is selected from the group consisting of sodium, potassium, lithium, rubidium, cesium, calcium, barium, strontium, and magnesium.

4. The method of claim 2 further characterized in that said salt comprises sodium sulfate.

5. The method of claim 2 further characterized in that said solution is applied substantially uniformly over said one side of the sheet whereby to efiect substantially uniform dulling of the surface on said one side.

6. The method of claim 2 further characterized in that said solution is applied to only the light coated side of the sheet for dulling the same.

7. The method of claim 2 further characterized in that the amount of salt applied to said sheet is from about 100 to about 300 milligrams per square foot of treated surface.

8. The method of claim 2 further characterized in that said silicate comprises sodium silicate and said salt comprises sodium sulfate.

9.. The method of claim 2 further characterized in that said solution also contains a surface active agent.

10. The method of claim 2 further characterized in that said solution also contains, a non-ionic surface active agent. 7

a 11. In the continuous production of differentially coated tin plate by passing a continuous strip of base metal through an electrolytic tinning zone and therein effecting controlled electrolytic deposition of tin so that the opposite sides of the strip have relatively light ,and relatively heavy tin coatings, respectively, and thereafter continuously passing the diflferentially coated strip through a fusion zone at elevated temperature for fusing the electrodeposited tin, the method of providing an identifying marking on one side of the strip which comprises continuously passing the strip through a treating zone intermediate said tinning and fusion zones and therein spraying only one of the tin coated sides of the strip with an aqueous solution containing both an alkali metal silicate and a salt selected'from the group consisting of the alkalinous metal chlorides, bromides, iodides, sulfates, carbonates, nitrates, and chromates, said salt being stable at the elevated temperature of the fusion step, and the amount'and concentration of the solution applied being sufficient to provide at least 100 milligrams of said salt per square foot of treated surface whereby the effluent strip from said fusion zone is provided with a dull surface marking of materially reduced reflectance thereby distinguishing the opposite sides of the sheet from each other, and said silicate preventing loss of the salt from the sheet by decrepitation and spalling during the fusion step.

12. The method of claim 11 further characterized in that said solution is sprayed on said strip in said treating zone by electrostatic spray means.

13. The process of claim 2 further characterized in that said solution contains from about 0.5 to about 10 grams of said salt per cc. of solution and from about 0.75 to about 2 cc. of said silicate per 100 cc. of solu- 

1. IN THE PRODUCTION OF TIN PLATE BY ELECTROLYTICALLY DEPOSITING TIN ON A BASE METAL SHEET AND THEREAFTER HEATING THE SHEET TO AN ELEVATED TEMPERATURE FOR FUSING THE ELECTRODEPOSITED TIN, THE IMPROVEMENT WHICH COMPRISES APPLYING TO A TIN COATED SURFACE OF THE SHEET INTERMEDIATE THE ELECTRODEPOSITION AND FUSION STEPS AN AQUEOUS SOLUSELECTED FROM THE GROUP CONSISTING OF THE ALKALINOUS SELECTED FROM THE GROUP CONSISTING OF THE ALKALINOUS METAL CHLORIDES, BROMIDES, IODIDES , SULFATES, CARBONATES NITRATES AND CHROMATES, SAID SALT BEING STABLE AT THE ELEVATED TEMPERATURE OF THE FUSION STEP, AND THE AMOUNT AND CONCENTRATION OF THE SOLUTION APPLIED BEING SUFFICIENT TO PROVIDE AT LEAST 100 MILLIGRAMS OF SAID SALT PER SQUARE FOOT OF TREATED SURFACE WHEREBY THE TIN PLATE FOLLOWING THE FUSION STEP IS PROVIDED WITH A DULL SURFACE MARKING AND SAID SILICATE PREVENTING LOSS OF THE SALT FROM THE SHEET BY DECREPITATION AND SPALLING DURING THE FUSION STEP. 